The invention relates to a method for screening of multi-junction solar cells used in high intensity and high temperature environments, each of the solar cells comprising at least two pn-junctions stacked on top of each other.
Solar cells are large area devices characterized by the fact that their lateral dimensions usually in the range of a few centimeters and are orders of magnitude larger than their thickness, which is in the range of some 10 to 100 micrometers. Advanced solar cells very often feature a multi-junction layout in which several pn-junctions are stacked on top of one another. The bandgaps of the individual junctions decrease from top to bottom. Therefore, each junction is sensitive to only part of the spectrum, which greatly reduces thermalization losses and results in higher efficiencies.
A typical example is III-V triple junction solar cells as illustrated in cross section in FIG. 1. A single crystalline Germanium (Ge)-wafer 1, typically around 100 micrometers thick, is used as a growth substrate. By metal organic vapour phase expitaxy (MOVPE), the GaAs pn-junction 2 followed by a GaInP2 junction 3 as well as several support layers (such as tunnel diodes, back surface fields, etc.) are grown epitaxially on this substrate. With a front metallization 4 and a rear side metallization 5 as well as an activation of the Ge-wafer itself by diffusion (depicted with reference numeral 6) the configuration shown in FIG. 1 electrically resembles a series connection of three solar cells. The order of the pn doping (n on top) is not relevant.
Due to the large lateral dimensions of the solar cells, in practice very difficult to obtain MOVPE grown layers completely free of areas where the epitaxial growth is distorted. These areas are called growth defects. While a lot of these defects have a negligible electrical impact (cf. reference numeral 7) some of them denoted with reference numeral 8 can act as an ohmic or diode like shunt. In this context, a diode like shunt is a growth defect with a lower open circuit voltage (Voc) than the remaining cell area. These shunts dissipate power produced by the surrounding undisturbed cell area. In contrast, the growth substrate 6 can be assumed to be free of intrinsic defects.
A spatially resolved electrical characterization technique is necessary in order to detect these shunts in a given solar cell.
A known technique to characterize a solar cell locally is electroluminescence imaging. This procedure is illustrated in FIG. 2. The solar cell is forward biased with an external power supply 9 at a particular current 1. The electroluminescence radiation emitted by a given junction, e.g. pn-junction 2, is recorded with an appropriate detector, for example a Si or InGaAs CCD chip 10 in combination with appropriate filters 11. The emitted electroluminescence intensity is exponentially dependent on the local junction voltage V (cf. reference numeral 12) at any position xy (cf. reference numeral 13) on the solar cell. Due to this very sensitive voltage dependence, the location of growth defects can be easily identified. Even defects in junctions not imaged directly, e.g. pn-junction 3, appear with an inverse contrast in the imaged junction due to the electrical series connection. A decrease in the local voltage in one junction, for example, automatically leads to a higher voltage at the remaining junctions because the external voltage V between the front metallization 4 and the rear side metallization 5 is constant everywhere when series resistance effects are neglected. However, the defects cannot be classified according to their electrical impact.
A measurement concept based on electroluminescence imaging for III-V triple junction solar cells that allows classifying defects according to their electrical impact is disclosed in the article [C. G. Zimmermann, “Performance Mapping of Multijunction Solar Cells Based on Electroluminescence”, IEEE ELECTRON DEVICE LETTERS, Vol. 30, No.8,August 2009, , pages 825-827]. With this procedure the electroluminescence radiation is recorded in a spatially resolved manner by planar detectors with appropriate filters. This is schematically illustrated in FIG. 3 showing three detectors 10, 16, 17 and assigned filters 11, 14, 15. All detectors image the entire area of the solar cell. For the sake of simplicity, in FIG. 3 the detectors are illustrated parallel to each other. As an alternative, it would be possible to move the solar cell from one detector 10, 16 to the next detector 16, 17. It is also feasible to use a common detector for several subcells, i.e. the amount of pn-junctions 2, 3, 6 stacked on top of each other, and select the emission of one particular subcell by interchangeable filters.
In addition, the solar cell is forward biased at a range of different injection currents I1, I2, . . . , IN (cf. reference numeral 18) up to or beyond the short circuit current of the solar cell. As a result, the combined voltage of all three pn-junctions can be inferred in a spatially resolved manner from the combined electroluminescence intensities of all junctions. In order to be able to calculate the local diode properties of an equivalent single junction cell, the local current density also has to be known. Therefore the fact that the bottom cell pn-junction 6 is not grown by MOVPE but is based on a single crystalline Ge-wafer is used. Its electroluminescence properties and diode properties can be assumed to be spatially homogeneous. Under these circumstances the measured electroluminescence intensity of the bottom cell follows a power law dependence on the local current and the bottom cell electroluminescence images serve as a map of the current distribution. Within this framework, the local diode properties of the cell can be calculated and from them the amount of current Iop, i.e. operation current, produced or dissipated at any position xy (cf. reference numeral 13) on the solar cell at a given operating condition is inferred. Because it is necessary to acquire images with high dynamic resolution of all junctions at a range of injection currents, the data acquisition time is in the range of several minutes, typically 10 to 15 minutes.
Since several thousand solar cells are required for the solar array, this more precise method is too costly and time consuming to screen each of the solar cells in this way. Alternatively, a direct screening of the cells by exposing them to their intended high temperature high intensity environment followed by an electrical performance measurement poses the risk of introducing artificial damage, e.g. if the screening is performed in air.
Exemplary embodiments of the present invention provide a more effective method for screening of multi-junction solar cells that are operated in space.
According to the invention, a method for screening of multi-junction solar cells to be operated in a high sun intensity and high temperature (HIHT) environment is provided, wherein each of the solar cells comprises at least two pn-junctions stacked on top of each other and wherein one of the pn-junctions is a homogeneous pn-junction assumed to be free of intrinsic defects. The method comprises the steps of providing a number of solar cells to be screened for usability in a HIHT environment; for each solar cell of the number of solar cells, acquiring an electroluminescence (EL) image of the homogeneous pn-junction at a predefined biase current, and analyzing the spatial intensity distribution in the electroluminescence image to determine whether there are local intensity variations that possibly dissipate power in a HIHT environment; sorting the solar cells wherein solar cells having no local intensity variations in their electroluminescence image are put into a first group of solar cells and solar cells having at least one local intensity variation in their electroluminescence image are put into a second group of solar cells for further screening wherein solar cells of the first group are suitable for HIHT environment and solar cells of the second group are assumed to be potentially critical in a HIHT environment.
The method of the invention is based on the consideration that under normal operation conditions in space with temperatures of around 50° C. and an irradiance of one sun as detailed in the AM0 spectrum [Solar Constant and Air Mass Zero Solar Spectral Irradiance Tables, (American Society for Testing and Materials, Philadelphia, 1992), Vol. ASTM-E 490-73a.], local growth defects are of no concern. In contrast, in more extreme environments, characterized by higher sun intensities and higher temperatures (HIHT environment), solar cells may be destroyed electrically by local overheating, i.e. local hotspots. An HIHT environment is characterized, for example, by temperatures of around 250° C. and five times the AMO spectrum. These environmental parameters are typical for space missions to the inner planets of the solar system, e.g. to Mercury. It was discovered that all solar cells that failed had local areas where current was dissipated. Under the above mentioned operating conditions, the locally dissipated current results in a local temperature increase. This in turn lowers the relevant parameters of a defect, e.g. the open circuit voltage Voc even more with an even bigger current dissipation. Hence, the solar cell is eventually destroyed in a positive feedback loop. For solar arrays, it is critical to exclude any solar cell that is susceptible in this way in a HIHT environment, in particular because subsets of solar cells are interconnected in series (so-called cell string) to reach the desired bus voltage. Therefore, the loss of any solar cell can result in the loss of the entire string. In addition, the thermal effect of hot spots can result in an insulation failure of one solar array with even more serious consequences.
Based on this knowledge, the method for screening of multi-junction solar cells according to the invention uses a simplified screening process that in a first step just separates those solar cells where at least one local intensity variation in their electroluminescence image of the homogeneous pn junction has been found from those where no such local intensity variation occurs. It has been found that solar cells having no such local intensity variation in their electroluminescence image are not critical for use in a HIHT environment because no power is dissipated locally. The other solar cells might be critical and will therefore be subject to a more detailed screening process. As a result, the screening of a huge amount of solar cells can be carried out in shorter time and without the risk of introducing artificial damage compared to an exposition to a simulated HIHT environment as set out in the prior art. The screening method of the invention is therefore cost efficient and time saving.
According to an aspect, the step of analyzing the spatial intensity distribution in the electroluminescence image of the homogeneous pn junction comprises the steps of calculating of an average intensity Φav of the entire electroluminescence image or a sub-area thereof and calculating the standard deviation σ; comparing, for each pixel xy of the electroluminescence image, whether the intensity Φxy in this pixel is larger than a threshold intensity Φth, wherein Φth=(Φav+nσ, with n being a predefined constant; and assigning those pixel xy to form a local intensity variation where the intensity Φxy is larger than a threshold intensity Φth and that are adjacent to each other. It has been found sufficient that n=6 for a triple junction solar cell which are preferred to be used in space due to their high efficiencies.
According to a further aspect, the step of analyzing the spatial intensity distribution in the electroluminescence image is executed by an image processing system.
According to a further aspect, the homogeneous pn-junction is a single crystalline wafer, particularly a Germanium (Ge)-wafer, on which the at least one further pn-junction is grown. The growth of the at least one further pn-junction may be made with metal organic vapour phase epitaxy (MOVPE).
According to a further aspect, during the screening process solar cells are placed on a temperature controlled plate that is heated to a predefined temperature. In principle, the temperature chosen during the screening process is not relevant. However, room temperature during the screening process is most straight forward.
According to a further aspect, an electroluminescence image is acquired with a detector having a resolution of around 2000 pixel/cm2 and/or having a dynamic resolution of about 1:10000.
According to a further aspect, a set of filters is used to block electroluminescence radiations emitted by other pn-junctions than the homogeneous pn-junction. As a result, only the electroluminescence radiation emitted by the homogeneous pn-junction is detected and processed. This allows acquisition of an electroluminescence image of only the homogeneous pn-junction that is a measure of the local current density. When considering the local intensity variations in the spatial intensity, shunts in the at least one further pn-junction can be detected. Due to the electrical series connection of the subcells, i.e. the amount of pn-junctions, they affect the current distribution in all layers.
According to a further aspect, the predefined bias current is a forward current close to the short circuit current of the solar cell such that the solar cell emits sufficient electroluminescence radiation for acquiring the electroluminescence image of the homogeneous pn-junction of the solar cell and that it is not damaged by the bias current.
According to a further aspect, the screening method further comprises the step of screening the solar cells of the second group of solar cells. It further comprises the steps of acquiring an electroluminescence image of all pn-junctions of a respective solar cell and a range of predefined bias currents; for each solar cell of the solar cells of the second group, creating a spatial current distribution map of the current Iop produced by the solar cell at a given cell operating voltage, and identifying pixels xy of a group of adjacent pixels xy with a lowest current value; and sorting those of the solar cells into a third group of solar cells that are not suitable for a HIHT environment based on the lowest current value.
This second more detailed screening step considers the electroluminescence radiation of all pn-junctions of the screened solar cell. By analyzing the lowest current value of pixels or a group of adjacent pixels it is possible to make a decision whether or not the screened solar cell is suitable for a HIHT environment.
According to a further aspect, the step of identifying pixels xy with a lowest current value comprises the step of determining whether or not the lowest current value is negative. When the lowest current value is not negative, no current is dissipated and the solar cell is assorted to a fourth group of solar cells that are suitable for HIHT environment. In contrast, when the lowest current value is negative the second screening step comprises the steps of selecting a subset of solar cells covering the entire spread of negative Iop values; exposing all solar cells of the subset to a simulated HIHT environment; determining solar cells with degraded electrical performance; determining a threshold current being negative above which no electrical degradation occurs; and sorting solar cells with an operating current Iop smaller than a threshold current Ith (Iop<Ith) into the third group and assorting solar cell with Iop>Ith into the fourth group. After having carried out this last step of the screening method the screening process is finished because each of the solar cells is reliably classified as being suitable for a HIHT environment.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.